A cosmid gene library of Actibnomyces viscosus T14V was prepared in Escherichia coli to examine the expression of A. viscosus antigens and to gain insight into the structure of A. viscosus type 1 and type 2 fimbriae. Out of this library of 550 clones, 28 reacted in a colony immunoassay with antibodies aginst A viscosus cells. The proteins responsible for these reactions were identified in 3 clones. Clones AV1209 and AV2009 displayed nonfimbrial antigens with subunits of 40 and 58 kilodaltons, respectively. Clone AV1402 showed a 59-kilodalton protein that reacted with monospecific antibody against type 2 fimbriae and that comigrated with a subunit of type 2 fimbriae during sodium dodecyl sulfate-polyacrylamide gel electrophoresis. This indicates that AV1402 expresses a gene (fimA) for a subunit of A viscosus type 2 fimbriae. Mapping and shotgun subcloning experiments with BamHI showed that fimA was contained within the 9.7-kib HindIII B fragment of pAV1402. This fragment was subcloned into pUC13. Expression of p59 was strongly orientation dependent, which indicates that the Actinomyces promoter for fimA has little or no activity in E. coli. Further mapping revealed that fimaA exression in AV1402 was dedpendent upon the nearby P2 promoter of tet in the vector.